This invention relates to a sewing machine needle, particularly for rapidly operating sewing machines.
As a rule, current industrial sewing machines operate with a speed of 5000 stitches per minute or more. In some cases even 8,000 stitches per minute are exceeded.
Upon penetration of the needle into the material to be sewn (hereafter referred to as fabric), the needle opens a stitch hole and pulls the thread (sewing yarn) into the stitch hole. At very high stitch numbers per minute only a very short period of approximately 0.5 ms or less is available for opening the stitch hole. Such a period starts at the moment the needle point contacts the fabric and lasts until the needle eye passes through the fabric. Because of an almost explosive opening of the stitch hole, large friction forces are generated between the needle and the fabric. Therefore, relatively large penetration Forces are needed which, in turn, increase the energy requirement for the needle penetration.
Because of the large frictional forces produced between the needle and the fabric during opening of the stitch hole, heat is generated which may thermally damage the fabric. For example, synthetic fibers may be melted or fused in the region of needle penetration. Also, fibers or fiber bundles may be mechanically damaged by the explosive opening of the stitch hole. A further consequence is the frequent sewing yarn breakage.
Further, the sewn article, dependent on the fabric, may be adversely affected by the fusion of laminates and layers as well as dye or finishing materials. These problems may lead to a significantly deteriorated seam quality and an affected wearability of garments made of such fabrics.
The above-outlined disadvantages have led to the desideratum to reduce the size of the stitch holes. In most cases, however, this objective is limited by the given yarn thickness and the size of the sewing machine needle necessary for guiding the yarn. A certain sewing yarn thickness determines a certain minimum needle thickness. Further, the size of the needle eye increases similarly to the needle thickness. An additional problem encountered in sewing machine needles is the mechanical stress on the needles, particularly at high-speed operation. The rotating masses in the sewing machine may generate vibrations which particularly affect the unilaterally clamped sewing machine needles. Further, thread pull, excursion of the needle as it contacts the fabric and manipulations by the operating personnel produce additional external forces. These factors lead to the desideratum to use thick sewing machine needles which have a sufficient rigidity. To use, on the one hand, a possibly thin needle for ensuring a gentle handling of the fabric and for avoiding an excessive heat-up of the needle and to use, on the other hand, a possibly thick needle for ensuring a required stability in case of high sewing speeds are contradictory requirements.
U.S. Pat. No. 4,037,641 describes a sewing machine needle having a straight blade which, in the vicinity of the needle point, has a needle eye passing through the blade transversely to the longitudinal needle axis. In the axial direction the eye is bordered by two facing eye webs which lie essentially on the central longitudinal axis of the needle. The needle is provided with a thread trough which merges into the eye and extends towards the needle shank which is clamped to the sewing machine. On the side remote from the thread trough a further, short trough is provided which extends toward the needle point and which runs out flat in the immediate vicinity at the needle eye. On the same side of the needle a depressed blade portion adjoins the eye and extends in the direction of the needle shank. The eye web situated between the thread trough and the depressed portion and rounded on its side serves for pulling the thread into an opened stitch hole.
By virtue of the approximately central arrangement of the eye web with respect to the needle blade, during needle penetration into the fabric, the thread runs on one side of the needle in the needle trough while on the other side of the needle the thread is essentially situated in the depressed blade portion. The eye region of the needle is oriented transversely to the central axis and is dimensioned in the opening direction of the eye in such a manner that the opened hole in the fabric is relatively large. As a result, the above-described disadvantageous occurrences may take place.
As the sewing machine needle is pulled out of the fabric in the course of the return stroke of the needle, the thread runs in an S-shaped curve through the eye. The friction generated in this instance produces forces between the needle and the thread and thus causes a heat-up.